The present invention relates to an optical system for an optical information-recording/reproducing apparatus adapted for writing information to or reading information from recording media such as optical disks or magneto-optical disks.
A prior art optical system to be used in an optical information-recording/reproducing apparatus of the type contemplated by the present invention is shown schematically in FIG. 8. The optical system generally indicated by A in FIG. 8 consists of a semiconductor laser 1 emitting a light beam, a collimator lens 2 for collimating the emitted light beam, a shaping prism 3 for correcting the cross-sectional profile of the light beam, a half mirror prism 4, an objective lens 5 for forming a beam spot on a magnetooptical disk D, a half-wave plate 6, a condenser lens 7, a polarizing beam splitter 10 for splitting the output beam from the condenser lens 7 into two beams which are launched into a tracking error detecting photosensor 8 and a focusing error detecting photosensor 9, respectively, and a cylindrical lens 11 positioned in front of the focusing error detecting photosensor 9.
The tracking error detecting photosensor 8 performs tracking error detection by the "push-pull" method on the basis of the difference between the outputs from two adjacent light-receiving areas, whereas the focusing error detecting photosensor 9 performs focusing error detection by the astigmatic method on the basis of the difference between the sum of outputs from a pair of diagonal light-receiving areas in a 2.times.2 matrix array and the sum of outputs from the other pair of diagonal light-receiving areas.
In the optical system A described above, the half-wave plate 6 and the polarizing beam splitter 10 work in such a way that the rotation of the vibration plane caused by the magnetic Kerr effect is converted to changes in the intensity of light beams which are issued to the photosensors 8 and 9 so as to detect the recorded information.
FIG. 9 shows another prior art optical system for use in information recording/reproducing apparatus. In the optical system generally indicated by B, a polarizing beam splitter 20 with a wedge prism as shown in FIG. 10 is positioned in front of a condenser lens 10, and a photosensor 30 is a six-divided element comprising a focusing error detecting portion 31 and a tracking error detecting portion 32 as shown in FIG. 11. With this arrangement, as shown in FIG. 10, the S-polarization component of polarized light for detecting a focusing error component is reflected from the interface 21 between a triangular prism and the wedge prism whereas the P-polarization component for detecting a tracking error component is reflected from the back surfaces 22 and 23 of the wedge prism as two separate beams. The reflected light components are directed to the focusing and tracking error detecting portions, respectively, of the photosensor 30.
When the intensity distribution of parallel light beam being launched into the objective lens in the prior art optical systems A and B is offset on account of variations in the optical axis of output light from the semiconductor laser used as a light source, said optical axis is brought into alignment with the ideal optical axis of the system by adjusting the angle between the laser and the laser mounting plate using a wedge-shaped correcting element. However, not only does this practice involve considerable difficulty in attaining precise adjustment but also many correcting elements must be provided to compensate for various offset angles of the optical axis of output laser light.
The offset in the distribution of light intensity appears as a push-pull offset in the tracking error component when the light beam reflected from the optical disk is focused on the photosensor and the resulting difficulty in precise tracking control causes deterioration in the signal.
When a wedge prism is used to form two separate beams for tracking error detection as in the prior art optical system B, a positional offset in the wedge prism upsets the balance between the quantities of the separated P-polarization components of light, causing a tracking offset. In order to avoid this problem, the prism and the photosensor must be positioned with very high precision. Further, if the photosensor 30 is moved with a view to correct only the tracking error, an offset occurs in the focusing error, and for this reason, the tracking offset cannot be adjusted by changing the position of the photosensor.